Stents are widely used for a significant variety of endoluminal treatments of patients. They may be used by themselves or as part of another medical device, such as a stent graft, a vena cava filter, an occluder, and so on.
Stents need to be able to maintain a lumen open as well as to press against the lumen wall with sufficient force to hold the stent in place over long periods of time and often indefinitely. Balloon expandable stents will typically be expanded to a diameter slightly larger than the normal lumen diameter; whereas self-expanding stents are chosen to have a relaxed diameter which is greater than the normal diameter of the lumen, such that they maintain a constant opening pressure against the vessel walls.
There is a compromise between the pressure applied by a stent and the integrity of the vessel, in that too great a pressure can lead to damage to the vessel wall, whereas too low a pressure can lead to failure of the stent to hold the vessel sufficiently open and failure to anchor reliably to the vessel. Whilst in most instances this compromise can be adequately managed in a patient, difficulties arise in cases where the shape or dimensions of the lumen are not consistent over time. One such instance is in the case of a lumen which expands over time, such as in a growing child, and/or in the case of severe injury such as an aortic transection following accident. Thus, a stent or stented medical device implanted into the patient to effect a medical treatment at a given point in time may become too small, that is narrow, as the patient's lumens heal and/or otherwise grow with age. Such change in the patient's lumen may lead to consequential risk of loss of anchoring of the medical device in the lumen and possible migration through the patient's vasculature; in other circumstances artificial constriction of the vessel as a result of device ingrowth into the vessel wall, thereby preventing natural expansion of the vessel.
It is not practicable or ideal to use a large diameter stent given the potential risk of damage to the vessel wall while the vessel has a smaller natural diameter, and it is equally not practicable to try to constrict the diameter of the vessel over time. As a result, it is often necessary to contemplate removal of the stent or other medical device after widening of the vessel, which involves a second medical procedure and frequent investigations up to that point. Even so, such a medical procedure may be difficult or not possible due to ingrowth of the device.
It is possible, especially in a growing patient or following severe vessel damage due to accident for instance, that the implantation of the medical device need only be temporary as growth/healing itself or a change in the patient's lifestyle or medical condition obviates the further need for the function of the medical device.
It is not just in patients who may experience changing lumens that it may be desirable to remove the stent or other medical device after a treatment period. Once the medical condition has been successfully treated, it may be desirable to remove the device given that this will continue to apply unnatural forces to the vessel walls. However, as explained above, ingrowth of the device into the tissue of the vessel walls or movement thereof can make removal of the device difficult. For this purpose, bioresorbable or biodegradable medical devices have been proposed, in which at least a part of the device is made of materials which will degrade over time, either into the blood stream or into the vessel tissue.
It is, however, not a simple task to replace parts of a medical device with bioresorbable or biodegradable material since this can lead to a change in the performance characteristics of the device. For instance, a self-expanding stent has a given springiness and is thus able to apply a given expansion or opening force to a vessel, normally over a variety of vessel diameters and vessel shapes, which cannot be emulated by biodegradable or bioresorbable materials.
Examples of prior art stents and other medical devices can be found, for instance, in U.S. Pat. No. 6,053,940, US-2005/228,486, US-2006/0,004,436, US-2010/0,249,902 and US-2011/0,166,640.